Stable water splitting using photoelectrodes with a cryogelated overlayer

Hydrogen production techniques based on solar-water splitting have emerged as carbon-free energy systems. Many researchers have developed highly efficient thin-film photoelectrochemical (PEC) devices made of low-cost and earth-abundant materials. However, solar water splitting systems suffer from short lifetimes due to catalyst instability that is attributed to both chemical dissolution and mechanical stress produced by hydrogen bubbles. A recent study found that the nanoporous hydrogel could prevent the structural degradation of the PEC devices. In this study, we investigate the protection mechanism of the hydrogel-based overlayer by engineering its porous structure using the cryogelation technique. Tests for cryogel overlayers with varied pore structures, such as disconnected micropores, interconnected micropores, and surface macropores, reveal that the hydrogen gas trapped in the cryogel protector reduce shear stress at the catalyst surface by providing bubble nucleation sites. The cryogelated overlayer effectively preserves the uniformly distributed platinum catalyst particles on the device surface for over 200 h. Our finding can help establish semi-permanent photoelectrochemical devices to realize a carbon-free society.

P exp is generated due to the elasticity of the overlayer as follows: where E overlayer is Young's modulus of the overlayer 11,12 .In the case of softening of the cryogelated overlayer, E overlayer is set to decrease linearly with time.
The partial volume of the bubble escapes through the surface pore of the cryogelated overlayer when P bubble (t) becomes larger than the capillary pressure P cap (t), which is calculated as follows: When no fracture occurs, ξ pore,base (t) is calculated using the following equation: When P bubble (t) is higher than the critical value P fracture , the pores at the overlayer surface can be coalesced by the local fracture.Thus, ξ pore,base (t) is set to increase as follows: ξ pore,base (t) = λ pore ×ξ pore,base (t-t step ) , where λ pore represents the degree of pore coalescence caused by fracturing.A higher value of λ pore indicates that a larger pore is formed by the overlayer fracture.
The flow rate of the escaping air from the deflating rubber balloon can depend on the volume and pressure of the air and the radius of the inlet of balloon 13 .We assumed that the volume of the bubble escaping from the pore V escape is a function of the volume and pressure of the bubble and the pore size.
In addition to the fractured pore, the trapped gas bubble should pass through the internal micropores as observed in Supplementary Fig. 17a.The gas transport rate is decreased by the path length which is increased by the cryogel thickness due to the resistance by the internal pores of the cryogel according to Poiseuille' Law 14 .The elastic deformation of micropores can also increase the resistance for the gas transport.Therefore, the effective path length would be determined by not only the geometry of the pores but the elastic properties of the cryogel.
The V escape is calculated as follows: , where A 0 , α, β, δ, and  are fitting parameters. path is a normalized effective path length.
Consequently, the volume of the bubble immediately after partial escape is as follows: V bubble (t) = V bubble (t-t step )+t step × V gas ̇− V escape (11)

Supplementary Fig. 2 .
photocathodes with and without the cryogel overlayer.(a) The current density (Jph)-time profile of Sb2Se3 photocathodes with and without the cryogelated overlayer measured via chronoamperometry at 0 VRHE under 1-sun illumination during 0 to 0.05 h.The devices incorporating cryogels with thicknesses of 400 μm, 800 μm, and 1200 μm were referred to as CG-400μm, CG-800μm, and CG-1200μm, respectively.The device without the cryogel was denoted as no cryogel.The black, blue, red, and green solid lines represent the Jph-time profile of no cryogel, CG-400μm, CG-800μm, and CG-1200μm, respectively.(b) The incident photon-to-current conversion efficiency (IPCE) spectra at 0 VRHE (solid line) and integrated current (dotted line) for no cryogel (black), CG-400μm (blue), CG-800μm (red), and CG-1200μm (green).The integrated IPCE values for all devices using the solar AM 1.5 spectrum were similar to the initial values obtained prior to Jph drop.The integrated IPCE value, calculated by integrating the values across all wavelength ranges, did not significantly change by the cryogel coating.For CG-400µm, CG-800µm, and CG-1200µm, the IPCE characterization was conducted at the freshly made state before the initial Jph drop occurred.

Table 1 . Device components and operation duration photocathodes composed of Pt catalyst, TiO2 layer, and low-cost thin-film light absorber.
3.